When light passes through a transparent medium, the light may be scattered in all directions. Two common light scattering phenomena are Rayleigh scattering and Raman scattering. In Rayleigh scattering, the light is scattered by molecules whose dimensions are smaller than the wavelength of radiation. The blueness of the sky, which results from the greater scattering of shorter wavelengths of the visible spectrum, is an example of Rayleigh scattering.
In Raman scattering, the wavelength of the scattered light is shifted from the wavelength of the incident light. The exact shifts in wavelength depend on the chemical structure of the medium or sample scattering the light. Raman lines having wavelengths higher than the incident wavelength are referred to as anti-Stokes lines and those having wavelengths lower than the incident wavelengths are referred to as Stokes lines. The intensities of Raman lines can be 0.001% or less when compared to the intensity of the incident light. Thus, detection of Raman scattering remains difficult.